1. Field of the Invention
The invention, in general, relates to sensor systems wherein active elements are mounted on a moving platform or carrier, and particularly to a data correction arrangement therefor.
2. Description of the Prior Art
In coherent electromagnetic and acoustic sensing systems operated from moving vehicles, small perturbations of the vehicle from uniform motion produce degrading amplitude, phase, and frequency modulation of the received signal. In many applications, it is impractical to control the vehicle with sufficient accuracy and smoothness to maintain these disturbances below acceptable levels and means must be provided to measure them and remove their effects from the received signal. This measurement instrumentation includes rate gyros, position gyros, and accelerometers and is generally very complicated and generally requires extreme precision of measurement. For use in an underwater sonar system where, due to the relatively slower speed of propagation of acoustic waves compared to electromagnetic waves and/or due to the relatively slower speeds of ships and submarines compared to land vehicles and aircraft, an order of magnitude greater precision is required, and a question arises as to whether sufficient accuracy is even attainable.
Instrumentation equipment consisting of rate gyros, position gyros, and accelerometers with associated mountings, motors, and electronics and associated computing equipment providing inertial measurement of vehicle perturbations from uniform motion and removal of the effects of such motion from coherent electromagnetic and acoustic sensing systems operating from moving vehicles will be referred to hereinafter as motion compensation equipment.
A general class of electromagnetic and acoustic sensing systems which normally requires motion compensation equipment is synthetic aperture radar and sonar mapping systems. In such systems, very fine angular resolution for imaging or resolving reflecting objects is achieved by sequentially generating a long effective aperture along the vehicle motion path. In such systems, the physical aperture (an antenna for a radar system or a transducer for a sonar system) transmits and receives reflections periodically along the vehicle path which are stored and combined by data processing means. In such systems, in order to achieve accurate results, the carrier of the sensor should not deviate more than a small fraction of a wavelength from uniform motion or, more generally, from a precisely defined course. In actual practice, such a stringent requirement is usually impractical and accordingly, motion compensation equipment is utilized to determine carrier deviations and correct the received signals to compensate for the deviations.
A second general class of electromagnetic and acoustic sensing systems which may require motion compensation equipment is MTI (moving target indication) radar and sonar. In MTI radar and sonar, reflections from reflectors with radial motion relative to the carrier of the sensor are discriminated from reflections from terrain and fixed reflectors by virtue of the Doppler frequency shift of the former. In MTI systems, signals reflected from terrain and fixed reflectors are suppressed by a cancellation filter means which eliminates or reduces signals with little or no Doppler shift. One type of cancellation filter involves subtracting reflected signals received from sequential transmissions separated by a time interval sufficiently long to allow signals from moving reflectors with Doppler shifts to change significantly between these transmissions. In this two-signal type of MTI system, the difference between successive signals from fixed reflectors is then very small and such signals are cancelled while the difference from moving reflectors with Doppler shifts is appreciable; and this difference can be detected, displayed, or subjected to further signal processing means. In addition to MTI systems employing the two-signal cancellation filter described above, MTI systems and cancellation filter means may exist in a variety of alternative designs, utilizing more than two signals, employing feedback, and incorporating adaptive features, for example.
Perturbations of the carrier from uniform motion in MTI systems impose Doppler shifts on signals reflected from terrain and fixed reflectors which may degrade the effectiveness of the cancellation filter operation. In such cases, motion compensation equipment may be employed to correct the received signals and compensate for such motion perturbations. A further source of degradation in MTI systems operated from moving vehicles in Doppler shifts imposed on signals reflected from terrain and fixed reflectors due to the finite width of the illuminating beam. These Doppler shifts occur because components of the carrier's uniform motion are different for reflectors at different angles within the beam, and they cause the spectrum of signals reflected from terrain and fixed reflectors to have an appreciable width.
A design approach which has been employed to aid motion compensation equipment in SAR systems and to reduce MTI system degradation due to the motion induced Doppler spectrum width of the terrain return is the use of dual aperture displaced phase center principles. In a dual aperture displaced phase center radar or sonar, two receiving apertures are mounted on the carrier displaced from each other in the direction of uniform motion. One of the apertures may be used for transmission or a separate transmitting aperture may be employed. In a dual aperture displaced phase center radar or sonar, the period between two successive transmissions is adjusted so that the carrier movement between these transmissions is approximately equal to the displacement of the equivalent aperture centers (taking into account both transmission and reception), and by receiving sequentially on the forward and rear apertures, respectively, the effect of two transmissions and receptions from approximately a stationary point is achieved. Signals received from approximately a stationary point will have minimum Doppler shifts imposed on signals reflected from terrain and fixed reflectors due to the finite width of the beam; and thus this source of performance degradation in MTI radar and sonar is minimized. The phase difference between the successive signals received by a dual aperture displaced phase center radar or sonar will be sensitive to side motion (normal to the primary carrier motion and to the displacement of the two apertures) and to angular movement producing such side motions between transmissions. In MTI applications, these disturbances may degrade performance and may require independent measurement and compensation. In SAR system applications, the phase angle between successive signal receptions can be employed to yield a measurement of side motion which may be more accurate than that derived from inertial measuring instruments.
The present invention represents an extension of dual aperture displaced phase center principles to more than two displaced apertures and more than two transmissions and receptions, allowing the requirement for inertial measurement of side and angular motion to be relieved or eliminated in MTI applications and the relief or elimination of angular motion inertial measurements in SAR applications.